Thermal control means

ABSTRACT

A thermal actuating device including a bimetal element with an electrical heating member which is resiliently coupled thereto. The resilient coupling permits relative movement between the bimetal and the heater upon deflection by the former. The heater is preferably a substrator heater.

United States Patent UN I 1 George Ronald Shepherd: Reginald Pltllp Burnlmn; Donald Copage, all at Norvlelt. Norfolk. England 814,542

Apr. 9, 1969 July 13, 1971 Diamond I-l. Controls Llmlted Norwich. Nortollt. England Apr. 10. 1968 Great Britain In ventors Appl. No. Filed Patented Assignee Priority THERMAL CONTROL MEANS 9 Claims, 8 Driving Flgs.

U.S.Cl 337/]07 .....l'l0lh 6l/0l3 Field 0! Search 337/77, 102, I03, I07, 1477;338/307, 308, 309, 217,142; 219/511 Primary Examiner- Bernard A. Gilheany Assistant Examiner- Dewitt M. Morgan Att0rney- Parker, Carter & Markey ABSTRACT: A thermal actuating device including a bimetal element with an electrical heating member which is resiliently coupled thereto. The resilient coupling permits relative movement between the bimetal and the heater upon deflection by the former. The heater is preferably a substrator heater.

13" 5 7 j 1 Viz Fi 7 //11 P\\\\\\\&\ 3 1 THERMAL CONTROL MEANS The present invention relates to thermal actuating devices employing a bimetal element and an associated electrical heater for use in a switch, e.g. an energy regulator.

Thermal switches employing a bimetal element and an associated electrical heater are known in which the heater takes the form of a coil wound on the bimetal element. The bimetal element, cooperates with a switching member so that the latter is actuated, as a result of the flexure of the bimetal element about a flexure point thereof during temperature variations, to open or close electrical contacts for the supply of power to a load.

This sort of construction has several shortcomings, of which one of the most important has been found to reside in the rigidity of the coil, which tends not only to oppose the very effect which it is desired to produce, namely, the deflection of the bimetal element as a result of the heat dissipated by the coil, but which is also, and to some extent consequentially, responsible for various inaccuracies and deficiencies in the performance of the regulator as a whole, for example, due to variation of the coil behavior as a result of repeated flexure together with the bimetal element.

Objects of the preset invention are to overcome the aforementioned shortcomings.

The invention provides a thermal actuating device comprising a bimetal element, an electrical heater member, and a resilient coupling member for coupling the heater member and the bim tal element, whereby relative movement takes place between the bimetal element and the heater member at the coupling member upon heating and cooling of the bimetal element.

The resilient coupling member may comprise a locating member, such as a rivet, screw or eyelet which passes through holes in the heater member and the bimetal element, and a spring means such as a spring washer or helical spring held by the locating member so as to resiliently urge the heater member towards the bimetal element.

The resilient coupling member may also be a leaf spring fixed to the bimetal element and arranged to press in a dimple or groove in the heater element so as to resiliently urge the latter towards the bimetal element.

In a preferred embodiment the heater comprises a resistive film on a substrate. To maximize the bimetal deflection sensitivity, the film is advantageously shaped so that the distribu tion of Joule heat along the film when a current is passed therethrough is at its greatest in the neighborhood of the flexure point of the bimetal element.

A film with a Y-shaped end portion has been found to give particularly good results, where the ends of the arms of the Y extend to the flexure point of the bimetal element.

The invention will be further described with reference to the embodiments shown in the accompanying drawings, wherein:

FIGS. 1 to 6 illustrate various preferred methods of resiliently coupling a heater to a bimetal;

FIG. 7 shows a first shaped substrate heater, mounted on an E-form balanced bimetal; and

FIG. 8 shows a second shaped substrate heater.

in FIGS. l to 6, reference 1 denotes a flat bimetal and 2 a heater member, also of a flat construction and which may be quite rigid, resiliently coupled to the bimetal so that relative movement may occur between heater member and the bim etal at the coupling position.

In FIGS. 1, 2 and 4 the resilient coupling comprises a spring washer 3 which is urged against the heater member or the bimetal by a locating member 4 passing through holes in the bimetal and the heater member. in FIGS. 1 and 4 the locating member is a rivet or tubular eyelet, while in FIG. 2 it is a shouldered screw.

in the embodiment of HO. 3 the heater member and the bimetal are resiliently coupled by means of a leaf spring 5 clamped by a screw or bolt 6 at one end to bimetal l, the other end of spring 5 sitting in a dimple or groove 7 in the heater member and thereby locating the latter as well as resiliently urging it towards the bimetal.

in FIG. 5 the resilient mounting comprises a spring clip 8, the center of which passes through a hole in the heater member and is spot welded at 9 to the bimetal, arms of the clip embracing the heater member on either side of the hole and resiliently urging it towards the bimetal.

In FIG. 6 the resiliently mounting comprises a locating rivet 10 passing through the heater member and the bimetal and having outwardly turned end portions I! for locating a helical spring [2 through which the rivet also passes. Spring 12 is arranged to be in a state of compression, so that heater member 2 is resiliently urged towards bimetal l. The embodiment of FIG. 6 has been found superior to those of FIGS. 1 to 5.

Any tendency for the heater to rotate relative to the bimetal because of the flexibility of the coupling can be prevented by making the heater member (say) abut a block or plate 13 (FIG. 1) clamping the bimetal to the body or casing of the device.

As mentioned above, heater member 2 may be of a rigid construction, and advantageously comprises a resistive film on a substrate. The use of such a heater construction gives many advantages over the conventional wound coil heater.

For example, with coil heaters there is the immediate mechanical difficulty of winding the coil, which usually consists of a large number of turns of fine wire. This complicates the manufacturing process and is associated with a comparatively high reject ratio due to breakage of the wire during production.

Also, the coil must be wound as close as possible to the pivot point of the bimetal element, for clearly a certain amount of heat imparted to the bimetal element at the pivot point will deflect the working arm of the element more than the same amount of heat dissipated further along the arm from the pivot point. Failure to satisfy this condition may have a very adverse effect in some applications.

For example in domestic cookers it is desirable for energy to be supplied to the load (eg the hotplate), after the latter has reached its set temperature, in the form of a train of frequent short pulses rather than as a train of infrequent long pulses. This is especially the case at low set temperatures in the simmer" range. Application of energy as a long pulse infrequently repeated means that during the pulse the load temperature may reach a value much in excess of the nominal setting. This situation may result if the heater coil is disposed too far from the pivot point of the bimetal element; indeed, imprecise positioning of the coil may lead in some cases to the pulse duration being increased by a factor of two and more.

A further disadvantage of coil heaters is that the flexure of the bimetal element during use imposes strains on the coil wire, which may lead to breakage of the wire and consequent open-circuiting of the coil.

A substrate heater comprises an insulating substrate or base on which there is deposited a film constituting the resistive element as such, and terminals preferably of some noble metal.

With modern techniques it has been found possible to produce small-size resistors of the order of 20 kohm which are able to dissipate around 20 w./sq.in., these characteristics being suitable for the use of such heaters in energy regulators and thermal switches.

The film may have a simple rectangular form, but is ad vantageously shaped so that the heat distribution produced upon passage of a current through the film is at its maximum in the neighborhood of the pivot point of the bimetal, as discussed above.

Such a shaped substrate heater is shown at M in FIG. 7. It comprises terminals 15 and 22, the former also constituting a resilient coupling, and a resistive film 23deposited on a substrate 24.

The heater is mounted by terminal-coupling 15 to the center arm 16 of an E-form balanced bimetal [7. A balanced bimetal is one in which the net deflection is independent of ambient temperature variations. The terminal is located within a Ushaped nonconductive region "I, so that current is constrained to flow substantially as indicated by the dashed lines 19. The current density and, consequently. the heating is greatest in conducting arms 20, 2], 23, which can readily be located near the bimetal pivot P so that heat is supplied to the bimetal where it will have most effect.

The use of a shaped substrate heater is not confined to use with E-shaped bimetals, such a heater may be used with any form of bimetal which is conventionally heated by wire wound heater, for example. with a single strip of bimetal or with a U- shaped bimetal.

H6. 8 illustrates a substrate heater construction which was found to be particularly effective. Here and 26 are silver terminal tabs, denotes the Y-shaped resistive film on a substrate 31, and 27 is a nonconductive region defining two arms 28 and 29 for the passage of current via the straight end portion 32 of the silver terminal tab 25. The heater is resiliently coupled to a bimetal (not shown) via hole 33. This construction has the following two advantages. Firstly the ends of the arms 28 and 29 extend to very near the root of the bimetal giving heat utilization. Secondly the heat generated in the arms 28 and 29 per unit area is increased. The substrate heater construction shown in FIG. 8 may be advantageously in the embodiment shown in FIG. 6.

We claim:

I. A thermal actuating device for a switch, said actuating device comprising a bimetal element, an electrical heating member, a spring means, and a locating member for locating the spring means, the locating member having a stern portion which passes through passages in the bimetal element, the heater member and the spring means, said stern portion having locating extensions at its ends, whereby relative movement takes place between the bimetal element and the heater member at the said passages upon heating and cooling of the bimetal element. and the heater member and the bimetal element are urged together along the direction of the stern portion by the reaction of the spring means against a locating estension of the said stem portion.

2. A thermal actuating device as claimed in claim I in which the spring means is formed as a spring washer.

3. A thermal actuating device as claimed in claim I in which the spring means is formed as a helical spring.

4. A thermal actuating device as claimed in claim 1 in which the heater member is formed as a substrate heater.

5. A thermal actuating device as claimed in claim 5 in which the resistive film of the substrate heater has a nonconducting portion therein, whereby, when current is passed through the film, the .loule heat dissipated at certain parts of the film will be greater than at others.

6. A thermal actuating device as claimed in claim 5 in which the nonconducting portion is formed as a substantially spiral strip.

7. A thermal actuating device as claimed in claim 5 in which the nonconducting portion is surrounded by the resistive film which is in the form of a Y-shaped region.

8. A thermal actuating device as claimed in claim 7 in which the bimetal element has a flexure point and the ends of the arms of the Y-shaped region extend to said flexure point.

9. A thermal actuating device for a switch, said actuating device comprising a bimetal element having a flexure point and an electrical heater member which is coupled to said bimetal element and which is formed as a substrate heater having a Y-shaped resistive film, the arms of which extend to said flexure point of the bimetal element whereby, then current is passed through the film, the Joule heat dissipated at the part of the film adjacent to said tlexure point of the bimetal element will be greater than at other parts of the film. 

2. A thermal actuating device as claimed in claim 1 in which the spring means is formed as a spring washer.
 3. A thermal actuating device as claimed in claim 1 in which the spring means is formed as a helical spring.
 4. A thermal actuating device as claimed in claim 1 in which the heater member is formed as a substrate heater.
 5. A thermal actuating device as claimed in claim 5 in which the resistive film of the substrate heater has a nonconducting portion therein, whereby, when current is passed through the film, the Joule heat dissipated at certain parts of the film will be greater than at others.
 6. A thermal actuating device as claimed in claim 5 in which the nonconducting portion is formed as a substantially spiral strip.
 7. A thermal actuating device as claimed in claim 5 in which the nonconducting portion is surrounded by the resistive film which is in the form of a Y-shaped region.
 8. A thermal actuating device as claimed in claim 7 in which the bimetal element has a flexure point and the ends of the arms of the Y-shaped region extend to said flexure point.
 9. A thermal actuating device for a switch, said actuating device comprising a bimetal element having a flexure point and an electrical heater member which is coupled to said bimetal element and which is formed as a substrate heater having a Y-shaped resistive film, the arms of which extend to said flexure point of the bimetal element whereby, then current is passed through the film, the Joule heat dissipated at the part of the film adjacent to said flexure point of the bimetal element will be greater than at other parts of the film. 